diff --git a/TODO.md b/TODO.md
index dc0e070..2fadf23 100644
--- a/TODO.md
+++ b/TODO.md
@@ -4,14 +4,14 @@ survivor-like optimized for weak hardware. finding the performance ceiling first
 
 ## phase 1: sandbox stress test
 
-- [ ] create sandbox.zig (separate from existing game code)
-- [ ] entity struct (x, y, vx, vy, color)
-- [ ] flat array storage for entities
-- [ ] spawn entities at random screen edges
-- [ ] update loop: move toward center, respawn on arrival
-- [ ] render: filled circles (4px radius, cyan)
-- [ ] metrics overlay (entity count, frame time, update time, render time)
-- [ ] controls: +/- 100, shift +/- 1000, space pause, r reset
+- [x] create sandbox.zig (separate from existing game code)
+- [x] entity struct (x, y, vx, vy, color)
+- [x] flat array storage for entities
+- [x] spawn entities at random screen edges
+- [x] update loop: move toward center, respawn on arrival
+- [x] render: filled circles (4px radius, cyan)
+- [x] metrics overlay (entity count, frame time, update time, render time)
+- [x] controls: +/- 100, shift +/- 1000, space pause, r reset
 
 ## phase 2: find the ceiling
 
diff --git a/build.zig b/build.zig
index bb3b08b..c3d9359 100644
--- a/build.zig
+++ b/build.zig
@@ -32,6 +32,30 @@ pub fn build(b: *std.Build) void {
     const run_step = b.step("run", "run the game");
     run_step.dependOn(&run_cmd.step);
 
+    // sandbox executable
+    const sandbox_exe = b.addExecutable(.{
+        .name = "sandbox",
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/sandbox_main.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
+    });
+
+    sandbox_exe.root_module.addImport("raylib", raylib_dep.module("raylib"));
+    sandbox_exe.linkLibrary(raylib_dep.artifact("raylib"));
+
+    b.installArtifact(sandbox_exe);
+
+    const sandbox_run_cmd = b.addRunArtifact(sandbox_exe);
+    sandbox_run_cmd.step.dependOn(b.getInstallStep());
+    if (b.args) |args| {
+        sandbox_run_cmd.addArgs(args);
+    }
+
+    const sandbox_step = b.step("sandbox", "run the sandbox stress test");
+    sandbox_step.dependOn(&sandbox_run_cmd.step);
+
     // test step (doesn't need raylib)
     const test_step = b.step("test", "run unit tests");
 
@@ -40,6 +64,7 @@ pub fn build(b: *std.Build) void {
         "src/trig.zig",
         "src/terrain.zig",
         "src/game.zig",
+        "src/sandbox.zig",
     };
 
     for (test_files) |file| {
diff --git a/src/sandbox.zig b/src/sandbox.zig
new file mode 100644
index 0000000..bdcc0f1
--- /dev/null
+++ b/src/sandbox.zig
@@ -0,0 +1,280 @@
+// sandbox stress test
+// measures entity count ceiling on weak hardware
+
+const std = @import("std");
+
+pub const SCREEN_WIDTH: u32 = 1280;
+pub const SCREEN_HEIGHT: u32 = 1024;
+const CENTER_X: f32 = @as(f32, @floatFromInt(SCREEN_WIDTH)) / 2.0;
+const CENTER_Y: f32 = @as(f32, @floatFromInt(SCREEN_HEIGHT)) / 2.0;
+const RESPAWN_THRESHOLD: f32 = 10.0;
+const ENTITY_SPEED: f32 = 2.0;
+
+pub const Entity = struct {
+    x: f32,
+    y: f32,
+    vx: f32,
+    vy: f32,
+    color: u32,
+};
+
+pub const MAX_ENTITIES: usize = 100_000;
+
+pub const Entities = struct {
+    items: [MAX_ENTITIES]Entity,
+    count: usize,
+
+    pub fn init() Entities {
+        return .{
+            .items = undefined,
+            .count = 0,
+        };
+    }
+
+    pub fn add(self: *Entities, rng: *std.Random) void {
+        if (self.count >= MAX_ENTITIES) return;
+        self.items[self.count] = spawnAtEdge(rng);
+        self.count += 1;
+    }
+
+    pub fn remove(self: *Entities, n: usize) void {
+        if (n >= self.count) {
+            self.count = 0;
+        } else {
+            self.count -= n;
+        }
+    }
+
+    pub fn reset(self: *Entities) void {
+        self.count = 0;
+    }
+};
+
+pub fn update(entities: *Entities, rng: *std.Random) void {
+    for (entities.items[0..entities.count]) |*entity| {
+        // apply velocity
+        entity.x += entity.vx;
+        entity.y += entity.vy;
+
+        // check if reached center
+        const dx = entity.x - CENTER_X;
+        const dy = entity.y - CENTER_Y;
+        const dist = @sqrt(dx * dx + dy * dy);
+
+        if (dist < RESPAWN_THRESHOLD) {
+            // respawn at random edge
+            entity.* = spawnAtEdge(rng);
+        }
+    }
+}
+
+pub fn spawnAtEdge(rng: *std.Random) Entity {
+    // pick random edge: 0=top, 1=bottom, 2=left, 3=right
+    const edge = rng.intRangeAtMost(u8, 0, 3);
+    const screen_w = @as(f32, @floatFromInt(SCREEN_WIDTH));
+    const screen_h = @as(f32, @floatFromInt(SCREEN_HEIGHT));
+
+    var x: f32 = undefined;
+    var y: f32 = undefined;
+
+    switch (edge) {
+        0 => { // top
+            x = rng.float(f32) * screen_w;
+            y = 0;
+        },
+        1 => { // bottom
+            x = rng.float(f32) * screen_w;
+            y = screen_h;
+        },
+        2 => { // left
+            x = 0;
+            y = rng.float(f32) * screen_h;
+        },
+        3 => { // right
+            x = screen_w;
+            y = rng.float(f32) * screen_h;
+        },
+        else => unreachable,
+    }
+
+    // velocity toward center
+    const dx = CENTER_X - x;
+    const dy = CENTER_Y - y;
+    const dist = @sqrt(dx * dx + dy * dy);
+    const vx = (dx / dist) * ENTITY_SPEED;
+    const vy = (dy / dist) * ENTITY_SPEED;
+
+    return .{
+        .x = x,
+        .y = y,
+        .vx = vx,
+        .vy = vy,
+        .color = 0x00FFFF, // cyan
+    };
+}
+
+// tests
+
+test "Entity struct has correct size" {
+    // 5 fields: x, y, vx, vy (f32 = 4 bytes each) + color (u32 = 4 bytes)
+    // 20 bytes total as per plan
+    try std.testing.expectEqual(@as(usize, 20), @sizeOf(Entity));
+}
+
+test "Entity can be created with initial values" {
+    const entity = Entity{
+        .x = 100.0,
+        .y = 200.0,
+        .vx = 1.5,
+        .vy = -0.5,
+        .color = 0x00FFFF,
+    };
+
+    try std.testing.expectEqual(@as(f32, 100.0), entity.x);
+    try std.testing.expectEqual(@as(f32, 200.0), entity.y);
+    try std.testing.expectEqual(@as(f32, 1.5), entity.vx);
+    try std.testing.expectEqual(@as(f32, -0.5), entity.vy);
+    try std.testing.expectEqual(@as(u32, 0x00FFFF), entity.color);
+}
+
+test "Entities init starts empty" {
+    const entities = Entities.init();
+    try std.testing.expectEqual(@as(usize, 0), entities.count);
+}
+
+test "Entities add increases count" {
+    var entities = Entities.init();
+    var prng = std.Random.DefaultPrng.init(12345);
+    var rng = prng.random();
+
+    entities.add(&rng);
+    try std.testing.expectEqual(@as(usize, 1), entities.count);
+
+    entities.add(&rng);
+    try std.testing.expectEqual(@as(usize, 2), entities.count);
+}
+
+test "Entities remove decreases count" {
+    var entities = Entities.init();
+    var prng = std.Random.DefaultPrng.init(12345);
+    var rng = prng.random();
+
+    // add 5
+    for (0..5) |_| entities.add(&rng);
+    try std.testing.expectEqual(@as(usize, 5), entities.count);
+
+    // remove 2
+    entities.remove(2);
+    try std.testing.expectEqual(@as(usize, 3), entities.count);
+
+    // remove more than count
+    entities.remove(100);
+    try std.testing.expectEqual(@as(usize, 0), entities.count);
+}
+
+test "Entities reset clears all" {
+    var entities = Entities.init();
+    var prng = std.Random.DefaultPrng.init(12345);
+    var rng = prng.random();
+
+    for (0..10) |_| entities.add(&rng);
+    try std.testing.expectEqual(@as(usize, 10), entities.count);
+
+    entities.reset();
+    try std.testing.expectEqual(@as(usize, 0), entities.count);
+}
+
+test "spawnAtEdge creates entity on screen edge" {
+    var prng = std.Random.DefaultPrng.init(12345);
+    var rng = prng.random();
+
+    // spawn several and check they're on edges
+    for (0..20) |_| {
+        const entity = spawnAtEdge(&rng);
+        const on_left = entity.x == 0;
+        const on_right = entity.x == @as(f32, @floatFromInt(SCREEN_WIDTH));
+        const on_top = entity.y == 0;
+        const on_bottom = entity.y == @as(f32, @floatFromInt(SCREEN_HEIGHT));
+
+        try std.testing.expect(on_left or on_right or on_top or on_bottom);
+    }
+}
+
+test "spawnAtEdge velocity points toward center" {
+    var prng = std.Random.DefaultPrng.init(12345);
+    var rng = prng.random();
+
+    for (0..20) |_| {
+        const entity = spawnAtEdge(&rng);
+
+        // after one step, should be closer to center
+        const dist_before = @sqrt((entity.x - CENTER_X) * (entity.x - CENTER_X) +
+            (entity.y - CENTER_Y) * (entity.y - CENTER_Y));
+
+        const new_x = entity.x + entity.vx;
+        const new_y = entity.y + entity.vy;
+        const dist_after = @sqrt((new_x - CENTER_X) * (new_x - CENTER_X) +
+            (new_y - CENTER_Y) * (new_y - CENTER_Y));
+
+        try std.testing.expect(dist_after < dist_before);
+    }
+}
+
+test "spawnAtEdge velocity has correct speed" {
+    var prng = std.Random.DefaultPrng.init(12345);
+    var rng = prng.random();
+
+    const entity = spawnAtEdge(&rng);
+    const speed = @sqrt(entity.vx * entity.vx + entity.vy * entity.vy);
+
+    try std.testing.expectApproxEqAbs(ENTITY_SPEED, speed, 0.001);
+}
+
+test "update moves entities by velocity" {
+    var entities = Entities.init();
+    // manually place an entity
+    entities.items[0] = .{
+        .x = 100.0,
+        .y = 100.0,
+        .vx = 2.0,
+        .vy = 3.0,
+        .color = 0x00FFFF,
+    };
+    entities.count = 1;
+
+    var prng = std.Random.DefaultPrng.init(12345);
+    var rng = prng.random();
+
+    update(&entities, &rng);
+
+    try std.testing.expectApproxEqAbs(@as(f32, 102.0), entities.items[0].x, 0.001);
+    try std.testing.expectApproxEqAbs(@as(f32, 103.0), entities.items[0].y, 0.001);
+}
+
+test "update respawns entity at edge when reaching center" {
+    var entities = Entities.init();
+    // place entity very close to center (within threshold)
+    entities.items[0] = .{
+        .x = CENTER_X + 1.0,
+        .y = CENTER_Y + 1.0,
+        .vx = -1.0,
+        .vy = -1.0,
+        .color = 0x00FFFF,
+    };
+    entities.count = 1;
+
+    var prng = std.Random.DefaultPrng.init(12345);
+    var rng = prng.random();
+
+    // after update, entity moves to center and should respawn
+    update(&entities, &rng);
+
+    // should now be on an edge
+    const entity = entities.items[0];
+    const on_left = entity.x == 0;
+    const on_right = entity.x == @as(f32, @floatFromInt(SCREEN_WIDTH));
+    const on_top = entity.y == 0;
+    const on_bottom = entity.y == @as(f32, @floatFromInt(SCREEN_HEIGHT));
+
+    try std.testing.expect(on_left or on_right or on_top or on_bottom);
+}
diff --git a/src/sandbox_main.zig b/src/sandbox_main.zig
new file mode 100644
index 0000000..29bef47
--- /dev/null
+++ b/src/sandbox_main.zig
@@ -0,0 +1,130 @@
+// sandbox stress test entry point
+// measures entity count ceiling on weak hardware
+
+const std = @import("std");
+const rl = @import("raylib");
+const sandbox = @import("sandbox.zig");
+
+const SCREEN_WIDTH = sandbox.SCREEN_WIDTH;
+const SCREEN_HEIGHT = sandbox.SCREEN_HEIGHT;
+
+// colors
+const BG_COLOR = rl.Color{ .r = 10, .g = 10, .b = 18, .a = 255 };
+const CYAN = rl.Color{ .r = 0, .g = 255, .b = 255, .a = 255 };
+
+pub fn main() !void {
+    rl.initWindow(@intCast(SCREEN_WIDTH), @intCast(SCREEN_HEIGHT), "lofivor sandbox");
+    defer rl.closeWindow();
+    rl.setTargetFPS(60);
+
+    var entities = sandbox.Entities.init();
+    var prng = std.Random.DefaultPrng.init(@intCast(std.time.timestamp()));
+    var rng = prng.random();
+
+    var paused = false;
+
+    // timing
+    var update_time_us: i64 = 0;
+    var render_time_us: i64 = 0;
+
+    while (!rl.windowShouldClose()) {
+        // controls
+        handleInput(&entities, &rng, &paused);
+
+        // update
+        if (!paused) {
+            const update_start = std.time.microTimestamp();
+            sandbox.update(&entities, &rng);
+            update_time_us = std.time.microTimestamp() - update_start;
+        }
+
+        // render
+        const render_start = std.time.microTimestamp();
+
+        rl.beginDrawing();
+        rl.clearBackground(BG_COLOR);
+
+        // draw entities as filled circles
+        for (entities.items[0..entities.count]) |entity| {
+            rl.drawCircle(
+                @intFromFloat(entity.x),
+                @intFromFloat(entity.y),
+                4,
+                CYAN,
+            );
+        }
+
+        // metrics overlay
+        drawMetrics(&entities, update_time_us, render_time_us, paused);
+
+        rl.endDrawing();
+
+        render_time_us = std.time.microTimestamp() - render_start;
+    }
+}
+
+fn handleInput(entities: *sandbox.Entities, rng: *std.Random, paused: *bool) void {
+    const shift = rl.isKeyDown(.left_shift) or rl.isKeyDown(.right_shift);
+    const add_count: usize = if (shift) 1000 else 100;
+
+    // add entities: = or +
+    if (rl.isKeyPressed(.equal) or rl.isKeyPressed(.kp_add)) {
+        for (0..add_count) |_| {
+            entities.add(rng);
+        }
+    }
+
+    // remove entities: - or _
+    if (rl.isKeyPressed(.minus) or rl.isKeyPressed(.kp_subtract)) {
+        entities.remove(add_count);
+    }
+
+    // reset: r
+    if (rl.isKeyPressed(.r)) {
+        entities.reset();
+    }
+
+    // pause: space
+    if (rl.isKeyPressed(.space)) {
+        paused.* = !paused.*;
+    }
+}
+
+fn drawMetrics(entities: *const sandbox.Entities, update_us: i64, render_us: i64, paused: bool) void {
+    var buf: [256]u8 = undefined;
+    var y: i32 = 10;
+    const line_height: i32 = 20;
+
+    // entity count
+    const count_text = std.fmt.bufPrintZ(&buf, "entities: {d}", .{entities.count}) catch "?";
+    rl.drawText(count_text, 10, y, 16, rl.Color.white);
+    y += line_height;
+
+    // frame time
+    const frame_ms = rl.getFrameTime() * 1000.0;
+    const frame_text = std.fmt.bufPrintZ(&buf, "frame:    {d:.1}ms", .{frame_ms}) catch "?";
+    rl.drawText(frame_text, 10, y, 16, rl.Color.white);
+    y += line_height;
+
+    // update time
+    const update_ms = @as(f32, @floatFromInt(update_us)) / 1000.0;
+    const update_text = std.fmt.bufPrintZ(&buf, "update:   {d:.1}ms", .{update_ms}) catch "?";
+    rl.drawText(update_text, 10, y, 16, rl.Color.white);
+    y += line_height;
+
+    // render time
+    const render_ms = @as(f32, @floatFromInt(render_us)) / 1000.0;
+    const render_text = std.fmt.bufPrintZ(&buf, "render:   {d:.1}ms", .{render_ms}) catch "?";
+    rl.drawText(render_text, 10, y, 16, rl.Color.white);
+    y += line_height;
+
+    // paused indicator
+    if (paused) {
+        y += line_height;
+        rl.drawText("PAUSED", 10, y, 16, rl.Color.yellow);
+    }
+
+    // controls help (bottom)
+    const help_y: i32 = @intCast(SCREEN_HEIGHT - 30);
+    rl.drawText("+/-: 100  shift+/-: 1000  space: pause  r: reset", 10, help_y, 14, rl.Color.gray);
+}